This invention relates to a process for producing an oriented composite polyolefin resin film. More specifically, it relates to a process for production with good molding characteristics of an oriented composite polyolefin resin film having good dimensional stability and appearance.
An opaque film is obtained by monoaxially or biaxially stretching or orienting at a suitable temperature a film of a thermoplastic resin in which an inorganic fine powder (hereinafter sometimes referred to as a filler) has been mixed. The resulting film is similar to pulp paper with respect to its opaqueness, whiteness, appearance and texture. It has been known that such a film can be used for a variety of uses of the conventional pulp paper, in the form of a sheet of monolayer structure consisting of the film, or a sheet of multilayer structure having the film as a surface layer.
The opaqueness and whiteness of the oriented thermoplastic resin film containing fillers is due to the fact that light is scattered on the surface and in the inside of the film. More specifically, this scattering of light is due to the facts that peeling between the resin and the filler particles takes place upon stretching the film to form minute interstices between the resin and the filler particles as the stretching proceeds, and minute voids containing therein the filler particles are formed in the interior of the film when the stretching is terminated, and also that minute cracks are produced on the surface because the formation of voids of a closed structure is not maintained in the vicinity of the film surface. Accordingly, the film is provided with the properties similar to pulp paper due to the presence of the above-mentioned surface cracks and interior voids.
However, when such an opaque film is subjected to a fabrication operation such as printing, folding, book-binding and bag making, the closed structures of the voids in the vicinity of the film surface is destroyed by the surface cracks. As a result, the filler particles which have been present on the film surface in a liberated or semi-liberated state are deposited onto the rollers, blankets or the like of the fabricating machine, which often results in trouble in the processing and markedly lowers the fabrication efficiency (hereinafter sometimes referred to as paper dust trouble).
Thus, in an opaque film having such a structure, it can be said that it is preferably that the surface layer have a low content of the filler as long as the film is printable or writable and also has paper-like opaqueness.
A film which satisfies such requirements is exemplified by a film of laminate structure which has been produced by laminating a layer having a relatively low content of a filler (paper-like layer) on the surface of a layer having a high content of a filler (substrate layer) (as disclosed in Japanese Patent Publication No. 42628/1976). In this known oriented film of laminate structure, however, both the paper-like layer and the substrate layer are biaxially oriented films. There are some problems in this film in that the surface has a pearl-like luster and also interlayer peeling between the two layers is apt to occur due to the fact that the paper-like layer is a biaxially oriented film. Moreover, the paper-like layer is not practicable for offset printing, although it may be practically used for gravure printing.
Such problems may be solved by the use of a laminate structure sheet in which the paper-like layer is a monoaxially stretched film and the substrate layer is a biaxially oriented film. According to our study, however, such a laminate structure sheet, which has been produced by using as the substrate layer a polyolefin resin having a melt index (MI) of 0.5 to 3 to provide the substrate layer with sufficient mechanical strength and stretching property and also employing a filler content of 20 to 68% to provide sufficient opaqueness, has been found to still have several problems. More specifically, the laminate structure sheet is produced by preparing a polyolefin resin sheet stretched in the machine operational direction for the substrate layer, melt-laminating a polyolefin resin for the paper-like layer on at least one surface of the stretched sheet, and then stretching the resulting laminated sheet in the cross-machine or transverse direction. However, because the substrate layer is a polyolefin resin having a low MI and a high filler content, the following defects are observed.
(1) A burned degraded resin is deposited and accumulated on the slit of a die for extruding the substrate sheet and produces streaks on the surface of an extruded sheet. As a result, the stretching cannot be conducted uniformly, and streaks of uneven opaqueness are observed when the resulting oriented composite film when light is passed therethrough.
(2) When the melt-extruded substrate sheet is cooled, shrinkage takes place around the nuclei of the filler particles to form a number of depressions on the sheet surface. As a result, a number of unstretched portions as large as rice grains are seen in the resulting oriented composite film (i.e., so-called "dimples").
(3) It is difficult to stretch a polyolefin resin sheet having a low MI and a high filler content. Thus, in addition to the above-mentioned formation of the depressions, nonuniform stretching is apt to take place.